Pump pressure control system

Abstract

A system for controlling the pressure output of an engine-driven centrifugal fire pump includes a discharge pressure sensor mounted in the discharge line of the pump. A governor varies the RPM of the engine in response to pressure changes detected by the discharge pressure sensor. An electronically controlled hydraulic relief valve responsive to the discharge pressure sensor operates simultaneously with the governor to provide improved pressure control in overpressure situations.

Having fully described and disclosed the instant invention and alternately preferred embodiments thereof in such clear and concise terms as to enable those skilled in the art to understand and practice the same, the invention claimed is: 
     
       1. A method for controlling the discharge pressure of an engine-driven centrifugal pump in a system including the pump,   an intake line coupled to the pump for receiving a liquid,   a discharge line coupled to the pump for discharging the liquid,   a discharge pressure sensor in the discharge line, for producing a first electronic signal proportional to the actual discharge pressure PA of the pump,   an over pressure relief valve in the discharge line, the overpressure relief valve being movable in a first direction away from a fully closed initial position, and a second direction toward the fully closed initial position, and   electronic control means electrically connected to the engine, the discharge pressure sensor and the overpressure relief valve, for varying the speed, the rate of change of speed, and ramp direction of engine,   the method comprising the steps of: a) selecting a desired discharge pressure P D  for the pump, a valve opening pressure P O  =P D  +ΔP 1 , a valve closing pressure P C  =P D  +ΔP 2  where P D  <P C  <P O , and a minimum desirable pressure P LOW  =P D  -ΔP 3  ;   b) electronically comparing the first electronic signal to a second electronic signal proportional to P D , and calculating an error E=P A  -P D  proportional to a difference between the first and second electronic signals;   c) actuating the electronic control means to set the engine ramp in a direction dependent on the sign and magnitude of E;   d) beginning to move the overpressure relief valve in the first direction, if closed, when E>ΔP 1 , and continuing to move the overpressure relief valve in the first direction until E<ΔP 1  ;   e) stopping movement of the overpressure relief valve in the first direction, if moving in the first direction, and holding the valve's position as long as ΔP 2  <E<ΔP 1  ;   f) beginning to move the overpressure relief valve in the second direction, if open, when E<P C , and continuing to move the overpressure valve in the second direction as long as 0<E<P C  ;   g) actuating the electronic control means to continuously adjust the rate of change in engine speed in the set direction, wherein the rate is calculated to maintain P LOW  <P A  <P O  without experiencing uncontrolled oscillations; and   h) continuously repeating steps b-g.     
     
     
       2. The method according to claim 1, wherein the step of actuating the control means to adjust the rate of change in engine speed occurs simultaneously with movement of the valve.